Field of the Invention
The present invention relates to a high proof stress 7000 series (Al—Zn—Mg) aluminum alloy extrusion material which is superior in bendability. The aluminum alloy extrusion material is suitable for complex shapes such as products shaped into shapes with a hollow part or a semi-hollow part with a high tongue ratio, and used for machine parts such as automobiles and two wheel vehicles.
Related Art
Generally, the structure of the aluminum alloy extrusion material includes a fiber-like crystalline form composed of subgrains and a crystalline form composed of recrystallized grains, and a recrystallized structure tends to be formed in a surface part which is especially highly processed and tends to accumulate strain due to characteristics of an extrusion process. The 7000 series aluminum alloy is highest in mechanical strength among all aluminum alloys, and an extrusion material thereof is a constructional material used in a part requiring mechanical strength and manufactured with an aging treatment for obtaining mechanical properties with high proof stress. In order to maintain high mechanical strength of the extrusion material, a major part of a metallographic structure is composed of a fiber-like structure, which requires suppression of recrystallization in manufacture. Therefore, a method of adding elements suppressing recrystallization (Zr, Mn, Cr, etc.), a method of extruding at a low temperature, a method of quickly chilling after extrusion, and the like have been conventionally used for suppressing formation of the recrystallized structure (Japanese Unexamined Patent Application Publication Nos. H10-298691, H8-120388, and H6-212338, referred to as Patent Documents 1 to 3, respectively).
Patent Document 1 discloses an Al—Zn—Mg aluminum alloy extruded shape for a motorcycle constructional member in which thickness of a recrystallized layer on a surface is reduced to no greater than 50 μm. It is described that it is possible for the extruded shape to undergo shaping such as a swaging process and a bending process in a high deformation region by: (a) preventing surface roughening of the shape by adding a recrystallization inhibitor such as Mn, Zr, and Cr to suppress formation of the surface recrystallized layer; and (b) reducing an amount of Mg to 0.20 to 0.50% by weight to improve elongation (paragraph 0009).
Patent Document 2 discloses an Al—Zn—Mg aluminum alloy hollow shape in which an interior structure is mainly a fiber-like structure and bending processability is improved by reducing thickness of a recrystallization structure on a surface part of the shape to smaller than 50 μm. It is disclosed that formation of the surface coarse recrystallized layer is suppressed by suppressing recrystallization by adding 0.2 to 0.5% Mn and quickly chilling the hollow shape immediately after extrusion by liquid nitrogen spraying.
Patent Document 3 discloses an Al—Zn—Mg aluminum alloy hollow shape having superior strength and shapability in which all of the metallographic structure is composed of a fiber-like structure. It is disclosed that recrystallization is suppressed by adding Zr, Mn, and Cr.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H10-298691
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H8-120388
Patent Document 3: Japanese Unexamined Patent Application, Publication No. H6-212338